Battery cathodes for improved stability

ABSTRACT

A lithium battery and method for fabricating the same are provided herein. The battery cathode comprises a carbon structure filled with a catalyst, such as palladium-catalyst-filled carbon nanotubes (CNTs). The carbon structure provides a barrier between the catalyst and the electrolyte providing an increased stability of the electrolyte during both discharging and charging of a battery.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 17/097,674, filed Nov. 13, 2020, which will issueas U.S. Pat. No. 11,038,160 on Jun. 15, 2021, which is acontinuation-in-part application of U.S. patent application Ser. No.16/901,398, filed Jun. 15, 2020, now U.S. Pat. No. 10,862,103, issuedDec. 8, 2020, which is a divisional application of U.S. patentapplication Ser. No. 16/235,049, filed Dec. 28, 2018, now U.S. Pat. No.10,734,636, issued Aug. 4, 2020, which claims the benefit of U.S.Provisional Application Ser. No. 62/612,036, filed Dec. 29, 2017, thedisclosures of each of which are hereby incorporated by reference intheir entirety, including all figures, tables, or drawings.

BACKGROUND

The search for high energy density batteries has motivated research inlithium-air batteries. Catalysts have been shown to improve both thebattery capacity and the recyclability of these batteries when used incathodes.

High energy density batteries have garnered much attention in recentyears due to their demand in electric vehicles. Lithium oxygen (Li—O₂)batteries have nearly 10 times the theoretical specific energy of commonlithium-ion batteries and in that respect have been regarded as thebatteries of the future. A typical Li—O₂ battery comprises a Li anode, aporous cathode open to oxygen, and a Li⁺ ion conducting electrolyteseparating the electrodes. A Li—O₂ battery stores energy via a simpleelectrochemical reaction (2Li+O₂⇄Li₂O₂) in which Li₂O₂ is deposited onthe surface of the cathode via a forward reaction (oxygen reductionreaction, ORR) during discharge and a backward reaction (oxygenevolution reaction, OER) takes place during charging to decompose Li₂O₂on the surface of cathode. Since the main discharge product (Li₂O₂) andother discharge/charge byproducts in Li—O₂ batteries are electricallyinsulating and not soluble in electrolytes, the structure and electronicconductivity of cathode materials have been critical factors indetermining the limiting capacity of Li—O₂ batteries. Carbonaceousmaterials such as carbon nanoparticles, carbon nanofibers, carbonnanotubes, graphene platelets, and other forms of carbons have beencommonly used as cathode materials in Li—O₂ batteries. Amongcarbon-based materials, carbon nanotubes (CNTs) have been widely used inLi—O₂ cathodes due to their high specific surface area, good chemicalstability, high electrical conductivity, and large accessibility ofactive sites. CNT (single-walled) have been used as cathode materials inLi—O₂ batteries and shown discharge specific capacities as high as 2540mAh·g⁻¹, which were obtained at a 0.1 mA·cm⁻² discharge current density.Although many research studies have been done to improve the performancemetrics of Li—O₂ batteries, they are still in their early stages andmany technical challenges have to be addressed before their practicalapplications.

The most common problems impeding the development of Li—O₂ batterieshave been low rate capability, poor recyclability, and low round-tripefficiency. All of these issues are originally stemmed from sluggishkinetics and the irreversible characteristic of the OER and ORRreactions which causes high overpotentials in the discharging/chargingprocess. Hence, increasing the efficiency of OER/ORR reactions andminimizing the overpotentials during the discharging/charging processhave been regarded as a meaningful approaches to overcome theaforementioned problems in Li—O₂ batteries.

Various additives have been explored to remedy this problem includingthe use of redox mediators. Redox mediators minimize charge polarizationby acting as charge carriers between the cathode and Li₂O₂ surface.Alternatively, different noble metals and metal oxide catalysts havealso been integrated in the cathodes of Li—O₂ batteries. The catalystmay influence the performance of Li—O₂ batteries by destabilizing theoxidizing species which decreases the charging overpotential. They mayalso increase the surface active sites and facilitate charge transportfrom oxidized reactants to the electrode which can also lead toformation of nanocrystalline Li₂O₂. However, it has been recently shownthat the catalyst on the oxygen cathode in Li—O₂ batteries is easilydeactivated due to continuous accumulation of discharge and chargeproducts upon cycling. It also has been reported that coarsening andagglomeration of catalyst upon charging/discharging reduces theefficiency of catalyst in Li—O₂ batteries. Platinum (Pt) and palladium(Pd) catalysts have been reported to promote Li₂O₂ oxidation at lowpotentials but also cause electrolyte decomposition resulting in theformation of Li₂CO₃ and thus deactivating the catalysts.

BRIEF SUMMARY

Embodiments of the subject invention provide Li-oxygen (Li—O₂) cathodesusing palladium-filled carbon nanotubes (CNTs) that increase thestability of the electrolyte during both discharging and charging of abattery. The combination of Li—O₂ cathodes and Pd-filled CNTs can beapplied to lithium-ion batteries, lithium-silicon, lithium-sulfur,lithium oxygen, as well as other non-lithium based batteries.

In an embodiment, a lithium battery can comprise: an anode including alithium metal; a cathode disposed on the anode; and an electrolytedisposed between the anode and the cathode, the cathode comprising acarbon cloth gas diffusion layer and a carbon structure with a metalcatalyst or a metal oxide catalyst, the metal catalyst or the metaloxide catalyst including a platinum group metal.

In another embodiment, a battery can comprise: an anode; a cathodedisposed on the anode; and a separator including an electrolyte anddisposed between the anode and the cathode, the cathode comprising acarbon cloth gas diffusion layer, a carbon structure, and a nanoparticlecatalyst, and the nanoparticle catalyst including a platinum groupmetal.

In yet another embodiment, a battery can comprise: an anode including alithium metal; a cathode disposed on the anode; and a separatorincluding an electrolyte and disposed between the anode and the cathode;a stainless steel tube disposed on the cathode; and a stainless steelrod disposed on the anode, the cathode comprising a carbon cloth gasdiffusion layer, a multi-walled carbon nanotube coated on the carboncloth gas diffusion layer, and a palladium nanoparticle catalyst coatedon a surface of the multi-walled carbon nanotube or filled in themulti-walled carbon nanotube, the separator being a polypropylene, andthe electrolyte including a lithium salt.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a Li—O₂ battery according to an embodiment of the subjectinvention.

FIG. 2 shows a plurality of carbon structures of a Li—O₂ batteryaccording to an embodiment of the subject invention.

FIG. 3(a) is a transmission electron micrograph of Pd-coated CNTs.

FIG. 3(b) is a transmission electron micrograph of Pd-filled CNTs.

FIG. 3(c) is a plot of a Raman spectrum of Pd-filled and Pd-coated CNTs.

FIG. 4 is a plot of the first discharge/charge capacity of pristine CNT,Pd-coated CNTs and Pd-filled CNTs at a constant current density of 250mAh.

FIG. 5 is a plot of cyclic voltammetry of pristine, Pd-filled, andPd-coated CNTs in range of 2-4.5 V vs Li/Li⁺, wherein all scans ratesare 1 mV·s⁻¹.

FIG. 6(a) is a plot of Raman spectrum after discharging and charging thebatteries for pristine, Pd-coated, and Pd-filled CNTs.

FIG. 6(b) is a plot of an x-ray diffraction patterns confirming thepresence of Li₂O₂ and Li₂CO₃ in Pd-coated and Pd-filled dischargedcathodes.

FIG. 7(a) is an image of a scanning electron micrograph of cathodesafter discharge for pristine CNTs.

FIG. 7(b) is an image of a scanning electron micrograph of cathodesafter discharge for Pd-coated CNTs.

FIG. 7(c) is an image of a scanning electron micrograph of cathodesafter discharge for

Pd-filled CNTs.

FIG. 8(a) is a plot of a chronopoteniometric test at 250 mA·¹ from OCVto 4.5 V for pristine, Pd-coated, and Pd-filled CNTs.

FIG. 8(b) is a plot of a linear sweep voltammetry of pre-dischargedPd-coated and Pd-filled CNTs under oxygen between OCV and 4.5 V vsLi/Li⁺ at a scan rate of 1 mV·s⁻¹, wherein the scale bars are 1 μm.

FIG. 9 is a plot of the cyclability of the Li—O₂ batteries for fixedcycle capacities of 500 mAh·g⁻¹ at a current density of 250 mA·g⁻¹ andwith voltage cutoffs of 2.04-4.5 V for pristine. Pd-coated, andPd-filled CNTs. The marker o denotes discharge capacity and the marker ●denotes charge capacity.

FIG. 10 is a plot of current (in milliamps (mA) versus potential (inVolts (V)), showing the cyclic voltammetry of lithium-sulfur (Li—S)batteries at a scan rate of 0.5 millVolts per second (mV/s). The (blue)curve with the lowest peak value at 2.45 V is for a Li—S batterycontaining pristine CNT cathodes; the (green) curve with the highestpeak value at 2.45 V is for a Li—S battery containing Pd-filled CNTcathodes; and the (red) curve with the second-highest peak value at 245V is for a Li—S battery containing platinum (Pt)-filled CNT cathodes.

FIG. 11 is a plot of specific capacity (in mAh/(g-s)) versus cyclenumber, showing the capacity retention due to cycling for the first 100cycles of Li—S batteries, cycled at 837.5 mA/(g-s) (C/2). The (blue)dots with the lowest value at 50 cycles are for a Li—S batterycontaining pristine CNT cathodes; the (green) dots with thesecond-highest value at 50 cycles are for a Li—S battery containingPd-filled CNT cathodes; and the (red) dots with the highest value at 50cycles are for a Li—S battery containing Pt-filled CNT cathodes.

DETAILED DESCRIPTION

Embodiments of the subject invention provide Li-oxygen (Li—O₂) cathodesusing palladium-coated and palladium-filled carbon nanotubes (CNTs). Itshould be appreciated by one of ordinary skill in the art that the CNTscan be replaced with various catalysts (for example, ruthenium, orplatinum-based catalysts) filled carbon structures, (for examplefullerenes, buckminsterfullerenes, or graphenes). Empirical data showsthat the full discharge of batteries in a 2-4.5 V range shows 6-foldincrease in the first discharge cycle of the Pd-filled over the pristineCNTs and 35% increase over their Pd-coated counterparts. The Pd-filledalso exhibits improved cyclability with 58 full cycles of 500 mAh·g⁻¹ atcurrent density of 250 mA·g⁻¹ versus 35 and 43 cycles for pristine andPd-coated CNTs, respectively. The effect of encapsulating the Pdcatalysts inside the CNTs leads to increased stability of theelectrolyte during both discharging and charging of the battery.Voltammetry, Raman spectroscopy, FTIR, XRD, UV/Vis spectroscopy andvisual inspection of the discharge products using scanning electronmicroscopy can be used to confirm the improved stability of theelectrolyte due to this encapsulation and that this approach could leadincreasing the Li—O₂ battery capacity and cyclability performance.

Multi-walled carbon nanotubes (MWCNTs) can be decapped by nitric acidsolution treatment and then 1 mM aqueous solution of PdCl₂ can be usedto swell 100 mg of decapped MWCNTs until a slurry is formed. Pd-coatedCNTs can also be prepared following the same procedure on untreatedcapped MWCNTs. Both slurries of Pd-coated and Pd-filled MWCNTs can bedried overnight at room temperature and calcinated in air at 350° C. for2 hours. Corresponding particles can then be hydrogenated in an ovenunder hydrogen gas to yield ˜5 wt % Pd nanoparticles. Cathodes can beprepared by coating a slurry of MWCNT (Pristine, Pd-filled andPd-coated)/PVDF (90/10 wt % in NMP) on a 0.5″ diameter carbon cloth gasdiffusion layer (CCGDL) followed by drying at 120° C. for 12 hours. Thecathodes can then be stored in an Ar-filled glove box to be used later.The typical loading of MWCNT can be 0.5±0.01 mg. All reported capacitiesin this application are reported per total mass of active cathode (CNTsand catalyst).

FIG. 1 shows a Li—O₂ battery according to an embodiment of the subjectinvention. Referring to FIG. 1 , the Li—O₂ battery 100 comprises acathode 200, an anode 400, and an electrolyte 300 disposed between thecathode 200 and the anode 400. The Li—O₂ battery 100 further comprises aconductive current collector 250 (e.g., a porous conductive currentcollector) disposed on the cathode 200 and a conductive currentcollector 450 disposed on the anode 400; the conductive currentcollector 250 disposed on the cathode 200 can be a tube, and theconductive current collector 450 disposed on the anode 400 can be a rod.The Li—O₂ battery 100 can be assembled by using a Swagelok type cell, inwhich the tube 250 is a stainless steel tube and the rod 450 is astainless steel rod. The electrolyte 300 can be formed as a separatorsoaked with an electrolyte, and the separator can be a polyolefin (e.g.,polypropylene). The anode 400 is formed by a lithium metal diskincluding a lithium metal.

The cathode 200 comprises a carbon structure with a metal catalyst ormetal oxide catalyst, wherein the metal catalyst or metal oxide catalystincludes a platinum group metal. The platinum group metal includes atleast one of ruthenium, rhodium, palladium, osmium, iridium, andplatinum. In an embodiment of the subject invention, a palladiumnanoparticle catalyst is coated on a surface of the carbon structure orfilled in the carbon structure. In addition, the cathode 200 furthercomprises the CCGDL, and the carbon structure having a platinum groupmetal catalyst is coated on the CCGDL. The cathode 200 includes a porousstructure open to an oxygen and the CCGDL has a woven structure.

FIG. 2 shows a plurality of carbon structures of a Li—O₂ batteryaccording to an embodiment of the subject invention. Referring to FIG. 2, the carbon structure of Li—O₂ battery includes at least one ofgraphene, fullerenes, amorphous carbons, and carbon nanotubes. Thecarbon nanotubes can be a single-walled carbon nanotube or amulti-walled carbon nanotube.

Referring to FIGS. 1 and 2 , the electrolyte 300 can be prepared byadding 1 mol·kg⁻¹ of LiTFSI salt (i.e., Lithium salt) into tetraethylglycol di-methyl ether (TEGDME) solvent. The lithium metal disc of theanode 400 is covered by an electrolyte soaked separator, MWCNT-CCGDL anda stainless steel mesh can be used as a current collector. Li—O₂batteries can be rested inside an Ar-filled glove box overnight beforeelectrochemical tests. All electrolyte preparation and cell assembly canbe performed inside an Ar-filled glove box (<1 ppm O₂ and <0.1 ppm H₂O).

The Li—O₂ batteries can be removed from the argon glove box and placedin the gastight desiccator filled with ultra-high purity oxygen gas(Airgas, purity>99.994%). The batteries can be rested under oxygen for 5hours before testing.

In certain embodiments of the subject invention, the CNTs can beprepared such that the Pd nanoparticles fill the carbon nanotubeswithout a Pd surface coating. CNTs can be decapped by introducing thenanotubes to an acid treatment. The decapped CNTs can then be rinsedwith water in order to remove any remaining acid treatment. The decappedCNTs can be dried and then immersed into a palladium salt solution andswelled until a slurry is formed. The CNTs can remain in the palladiumsalt solution until such time that the nanotubes are filled. The CNTscan then be dried, in a drying device, under oxygen to convert thepallidum salt to palladium oxide particles. The CNTs can then be rinsedto remove any debris remaining on the surface of the nanotubes. The CNTscan then be hydrogenated in a furnace to convert the palladium oxideinto palladium. The Pd-filled CNTs can then be stored, for example inArgon, until future use.

As materials of the Li—O₂ battery 100 according to the presentinvention, Palladium (II) chloride (PdCl₂, 59% Pd), Bis(trifluoromethane) sulfonamide (LiTFSI, purity>99.95%), tetraethyleneglycol dimethyl ether (TEGDME, purity>99.00%), N-Methylpyrrolidine (NMP,purity>97.00%), multi-walled carbon nanotubes (MWCNT, D=5-20 nm, L=5 μm,purity>95.00% carbon basis), Titanium (IV) oxysulfate (TiOSO₄) (≥29% Ti(as TiO₂) basis), and Lithium Peroxide (Li₂O₂) can be used. In addition,carbon cloth gas diffusion layer (CCGDL, thickness ˜300 μm), Lithiumfoil chips (purity>99.90%), a polypropylene separator (thickness ˜25μm), and Polyvinylidene fluoride (PVDF) can be also be used.

A Solartron 1470 battery tester can be used for galvanostaticdischarge/charge tests within a voltage range of 2.0-4.5 V at a currentdensity of 250 mA·g⁻¹. Voltammetry measurements are performed by anelectrochemical workstation (Gamry reference 600) at the rate of 1mV·s⁻¹ in the range of 2.0-4.5 V to investigate the catalytic behaviorof oxygen electrodes. All charge/discharge and electrochemical tests aremeasured in a temperature controlled environment at 25° C. Aftercharge/discharge cycling, the oxygen cathodes are recovered from thebatteries in the Ar-filled glove box, rinsed with acetonitrile and driedunder vacuum. Cathodes can be investigated by Raman spectroscopy(BaySpec's Nomadic, excitation wavelength of 532 nm), Fourier transforminfrared (FTIR) spectroscopy (JASCO FT-IR 4100), and Scanning electronmicroscopy (SEM) (JEOL 6330F). Bruker GADDS/D8 X-ray powder diffraction(XRD) with MacSci rotating Molybdenum anode (λ=0.71073) operated at 50kV generator and 20 mA current is also used to collect the diffractionpatterns. A parallel X-ray beam in size of 100 μm diameter is directedon to the samples and diffraction intensities are recorded on large 2Dimage plate during exposure time. Li₂O₂ is quantified in the cathodesafter discharge using a colorimetric method. Briefly, dischargedcathodes are first immersed in water then aliquots are taken and addedto 2% aqueous solution of TiOSO₄. Instantaneously a color changeoccurred and the absorbance spectra of the solutions are collected usinga UV-Vis spectrophotometer (Gamry UV/Vis Spectro-115E). The peakintensity at 408 nm is calibrated against solutions with knownconcentrations of Li₂O₂, in the range of 0.1 to 10 mg/ml and linearcalibration curve is obtained. Transmission Electron Microscopy(Phillips CM-200 200 kV) is also used to inspect the carbon nanotubes.

The cathodes of the Li—O₂ battery can comprise MWCNTs (pristine,Pd-coated and Pd-filled) coated on the woven carbon cloth gas diffusionlayer (CCGDL). Homogenous three-dimensional networks of carbon nanotubesover CCGDL yield high surface area with an open structure which improvesthe electronic contact during charging and discharging processes. FIGS.3(a) and 3(b) show TEM images of Pd-coated and Pd-filled CNTs,respectively. Referring FIG. 3(b), in Pd-filled cathodes, Pdnanocatalysts are formed in the inner tubular region of decapped CNTs.FIG. 3(c) shows the Raman spectra of the Pd-coated and Pd-filled CNTs.Referring to FIG. 3(c), D and G bands of Pd-filled and Pd-coated CNTsare identical in location and intensity ratios [I(G)/I(D)˜1.2]indicating that the decapped CNTs do not have high density of defects ontheir surface. The left peak corresponds to the D band and the rightpeak corresponds to the G band.

FIG. 4 shows a plot of the first discharge/charge capacity pf pristineCNT, PD-coated CNTs and Pd-filled CNTs at a constant current density of250 mAh. The first discharge and charge behaviors of the Pd-coated,Pd-filled and pristine CNTs batteries using 1 M LiTFSI in TEGDMEelectrolyte in the voltage window of 2.0-4.5 vs Li/Li+ at the constantcurrent density of 250 mA·g⁻¹ are shown in FIG. 4 . Referring to FIG. 4, the pristine CNTs show a first discharge capacity of 1980 mAh·g⁻¹compared to 8197 mAh·g⁻¹ and 11,152 mAh·g⁻¹ for the Pd-coated andPd-filled CNTs, respectively. The ˜6-fold discharge capacity improvementof Pd-coated CNTs over pristine CNTs resulted from improved ORR andincreased surface sites for lithium discharge product deposition due tothe presence of Pd nanocatalysts.

FIG. 5 shows a plot of cyclic voltammetry of pristine, Pd-filled, andPd-coated CNTs in range of 2-4.5 V vs Li/Li⁺, wherein all scans ratesare 1 mV·s⁻¹. Referring to FIG. 5 , Cyclic voltammetry (CV) measurementsconfirm higher ORR and OER currents for Pd containing CNTs over pristineCNTs. This also confirms retained catalytic activity of Pd whenencapsulated inside the CNTs, consistent with previously observations.In addition, the presence of Pd nanocatalysts in both Pd-filled andPd-coated CNTs shifts the onset of ORR peak of pristine CNTs from 2.8 Vto 2.9 V, showing enhanced cathodic activity. The Pd-filled CNTs alsodemonstrates a 36% increase in first discharge capacity over Pd-coated.In the FIG. 5 , an oxidation peak at ˜3.3 V is attributed to the OER andis shown to be more pronounced for the Pd-filled compared to Pd-coated.It is considered that the presence of Pd inside the CNTs strengthens theπ electron density on the CNT surface yielding homogeneously distributednucleation sites for Pd-filled CNTs compared to heterogeneouslydistributed nucleation sites for Pd-coated CNTs. This delocalization ofLi₂O₂ seeding sites contributes to intimate contact between Li₂O₂ andCNTs and helps promote the formation of high surface discharge products.At voltage exceeding 3.7 V vs Li/Li ⁺ Pd-coated CNTs show the highestrate of oxidation of electrolyte and electrolyte decomposition products.

FIG. 6(a) shows a plot of Raman spectrum after discharging and chargingthe batteries for pristine, Pd-coated, and Pd-filled CNTs. Referring toFIG. 6(a), Raman spectroscopic characterization on the dischargedcathodes shows Li₂O₂ formation at ˜790 cm⁻Raman shift for all dischargedcathodes. However, the Pd-coated CNTs cathode shows a pronounced Ramanshift peak at 1080 cm⁻¹, which corresponds to the electrolytedecomposition product Li₂CO₃. This peak is absent from the pristine CNTcathodes. This behavior confirms the observation from CV, indicatingreduced electrolyte stability due to the presence of Pd on Pd-coatedCNTs and enhanced electrolyte stability for Pd-filled compared toPd-coated CNTs. Following charging, Raman spectroscopic analyses on thecathodes shows efficient removal of the Li₂O₂ peaks from all cathodes,and a decrease in peak intensity of Li₂CO₃ peak for the Pd containingCNTs. The decrease in Li₂CO₃ was previously reported to be enabled incatalyst-containing cathodes. Li₂O₂ content in the cathode is quantifiedusing a colorimetric approach. The amounts of Li₂O₂ in the cathode aredetermined to be 11.6, 21.4, and 35.4 μmols for pristine, Pd-coated, andPd-filled CNT cathodes, respectively. These amounts correspond tocapacity yields of approximately 88%, 23%, and 37% of the experimentalcapacities recorded by the pristine, Pd-coated, and Pd-filled CNTcathodes, respectively.

In order to determine the molar ratio of Li₂O₂ in the dischargedcathodes, the cathodes are analyzed using FTIR. Using peak intensitiesratio at 600 cm⁻¹ (Li₂O₂) and 862 cm⁻¹ (Li₂CO₃), Pd-coated and Pd-filledcathodes have 19.3% and 33.2% Li₂O₂ by mole, respectively. By onlyconsidering the Li₂O₂ and Li₂CO₃ discharge species, this observation isin agreement with the UV-Vis quantification and further confirms thestabilizing effect of the encapsulation of Pd inside the CNTs comparedto coating the CNTs. The CV and Raman data also back up these claims,indicating that the electrolyte undergoes more decomposition in cellswith Pd-coated CNTs cathodes.

FIG. 6(b) show a plot of an x-ray diffraction patterns confirming thepresence of Li₂O₂ and Li₂CO₃ in Pd-coated and Pd-filled dischargedcathodes. Referring to FIG. 6(b), the presence of Li₂O₂ and Li₂CO₃ isadditionally confirmed by XRD.

FIGS. 7(a), 7(b), and 7(c) show images of a scanning electron micrographof cathodes after discharge for pristine CNTs, Pd-coated CNTs, andPd-filled CNTs, respectively. Referring to FIGS. 7(a), 7(b), and 7(c),the formation of discharge products is visually confirmed using scanningelectron microscopy on discharged cathodes. The discharge products(Li₂O₂) of the pristine CNTs are conformal around the cathode inrod-like structures with low porosity. The growth of layers in thismorphology often yields to passivation and blockage of oxygen to thecathode and eventually limits the discharge capacity and cycle life ofthe battery. The Pd-coated CNTs cathode as shown FIG. 7(b) shows someplatelet-shaped Li₂O₂ buried by thick conformal layer, suspected to beLi₂CO₃ as shown in Raman and FTIR measurements. In contrast, thePd-filled CNTs show nano-thin platelets of Li₂O₂ covering the cathode asshown FIG. 7(c). These platelets of Li₂O₂ yield high surface porositywhich in turn do not block the access to the CNTs and enhance theperformance.

In order to identify the synergy of electrolyte and Pd nanocatalysts,the oxidation stability limit of the electrolyte is determined using achronopotentiometric stability test and linear sweep voltammetry underoxygen atmosphere. Batteries using Pd-coated, Pd-filled and pristineCNTs are assembled and charged without prior discharging at constantcurrent density of 250 mA·s⁻¹ up to cutoff voltage of 4.5 V.

FIG. 8 shows higher capacity for Pd-coated compared to Pd-filled andpristine. In particular, FIG. 8(a) shows a plot of a chronopoteniometrictest at 250 mA·g⁻¹ from OCV to 4.5 V for pristine, Pd-coated, andPd-filled CNTs, and FIG. 8(b) shows a plot of a linear sweep voltammetryof pre-discharged Pd-coated and Pd-filled CNTs under oxygen between OCVand 4.5 V vs Li/Li⁺ at a scan rate of 1 mV·s⁻¹, wherein the scale barsare 1 μm. Since no discharge products existed, the capacities obtainedare attributed to electrolyte and cathode undesirable reactions. Thissuggests that Pd-filled CNTs promotes better stability than Pd-coatedCNT cathodes. This observation is also confirmed using linear sweepvoltammetry following a discharge showing increased OER peak intensityat 3.3-3.4 V and improved electrolyte stability above 3.7 V vs Li/Li⁺for Pd-filled vs Pd-coated CNTs in FIG. 5 . These results again supportprevious observations that encapsulating the Pd nanocatalyst helpsimprove the electrolyte stability during the operation of the Li—O₂battery.

FIG. 9 shows the cycling stability of Li—O₂ batteries based onPd-filled, Pd-coated, and pristine CNTs. Referring to FIG. 9 ,Galvanostatic discharge/charge cycling of Li—O₂ batteries at a currentdensity of 250 mA·g⁻¹ at a limited capacity of 500 mAh·g⁻¹ in a voltagewindow of 2.0-4.5 V vs Li/Li⁺ are conducted. Li—O₂ cells using Pd-filledCNTs show the highest discharge The cycling stability improvement in thecase of Pd-coated and Pd-filled CNTs is a result of previously confirmedOER/ORR improvement due to the Pd nanocatalysts. The cycling stabilityimprovement of Pd-filled CNTs over the Pd-coated CNTs is ultimatelycredited to the decrease in undesirable discharge/charge productsformation, e.g. Li₂CO₃, afforded by the encapsulation approach. Theinclusion of Pd catalyst in CNTs for use as cathode materials in Li—O₂batteries has been demonstrated. Using two modes of CNT loading, inside(Pd-filled CNTs) and outside (Pd-coated CNTs), shows that bothapproaches yielded significant improvement in full discharge capacitiesof the batteries while the Pd-filled cycled for 35% more cycles of 500mAh·g⁻¹ at current density of 250 mA·g⁻¹. The encapsulation ofnanocatalyst inside the CNTs improves the stability of the electrolyteby decreasing the formation of Li₂CO₃ compared to nanocatalyst-coatedCNTs. These observations are confirmed by voltammetry, Raman and UV/Visspectroscopy, FTIR, chronopotentiometry, electron microscopy, andcharge/discharge cycling. The Pd-filled CNTs and the Pd-coated CNTs arediscussed independently, but the CNTs that are coated with Pd outsideand filled with Pd inside at the same time can be used.

The cathodes for a lithium sulfur battery can comprise, for example,Pd-filled, Pt-filled, or pristine CNTs coated on a conductive layer(e.g., an aluminum, copper, silver, or gold layer such as an aluminum,copper, silver, or gold foil). The CNTs can be the same as describedabove for use in, e.g., an Li—O₂ battery cathode and can containnanocatalysts at a weight percentage of 0.1% to 20% by weight(preferably 0.5%-5% by weight). Sulfur can be added to the cathodes as,for example, elemental sulfur (S₈), lithium sulfide (Li₂S), and/orlithium polysulfide (Li₂S₈, Li₂S₆, Li₂S₄, Li₂S₂, etc.). The sulfur tocarbon weight ratio in the cathode can be, for example, 0.2-2.0(preferably 0.5-1.0). A polymeric binder such as polyvinylidene fluoride(PVDF), polytetrafluoroethylene (PTFE), or carboxymethyl cellulose (CMC)can be used. The film thickness can typically be in a range of 20micrometers (μm)-400 μm (preferably 100 μm-200 μm). The electrolyte caninclude, for example, a mixture of 1,3-dioxolane (DOL) and dimethylether (DME) with 1:1 by volume containing a lithium salt (e.g., lithiumbis(trifluoromethane)sulfonamide) at a predetermined concentration(e.g., a 1 M concentration). The electrolyte can also include asolid-electrolyte interface forming additive such as lithium nitrate(e.g., at 2%-10% by weight, preferably 5% or about 5% by weight). Theanode can be, for example, a lithium metal or a lithium metal film on ametallic current collector (e.g., copper).

In one example of cathode preparation for a lithium sulfur battery,CNT:sulfur:PVDF (25:70:5 by weight) are mixed in a dispersing solvent(e.g., n-methyl-2-pyrrolidone (NMP)) to form a slurry or suspension, andthe slurry is then coated on an aluminum foil (e.g., using a standardfilm coating technique such as a Mayer rod or a doctor blade). The filmis then dried (e.g., at room temperature for 2 hours) followed by drying(e.g., at 60° C.) under vacuum (e.g., for 12 hours). After drying thefilm is stored in an inert environment (e.g., an argon-filled glovebox)until cell assembly. For cell assembly the cathode films, lithium foil(purity>99.90%), a polyolefin separator (e.g., a propylene separator)with a thickness of, e.g., 25 μm or about 25 μm, and the DOL:DMEelectrolyte are used to assemble the coin or pouch cell. The cells aretypically rested (e.g., for 6 hours) prior to testing.

A Solartron 1470 battery tester can be used for galvanostaticdischarge/charge tests within a voltage range of 1.7-3.0 V at a currentdensity of 167.5 mA (C/10) to 3350 mA (2C) per gram of sulfur.Voltammetry measurements were performed by an electrochemicalworkstation (Gamry reference 600) at the rate of 1 mV/s in the range of1.7-3.0 V to investigate the catalytic behavior of the sulfur electrode.All charge/discharge and electrochemical tests were measured in atemperature controlled environment at 25° C. After charge/dischargecycling, the sulfur cathodes were recovered from the batteries in theAr-filled glove box, rinsed with dimethyl ether and dried under vacuum.Cathodes can be investigated by Raman spectroscopy (BaySpec's Nomadic,excitation wavelength of 532 nm), Fourier transform infrared (FTIR)spectroscopy (JASCO FT-IR 4100), and/or Scanning electron microscopy(SEM) (JEOL 6330F). Bruker GADDS/D8 X-ray powder diffraction (XRD) withMacSci rotating Molybdenum anode (k=0.71073) operated at 50 kV generatorand 20 mA current was also used to collect the diffraction patterns.Transmission Electron Microscopy (Phillips CM-200 200 kV) was also usedto inspect the carbon nanotubes. FIG. 10 shows a plot of cyclicvoltammetry of pristine CNTs, Pd-filled CNTs, and Pt-filled CNTs inrange of 1.6-2.9 V versus Li/Li+, wherein all scans rates were 0.5 mV/s.Referring to FIG. 10 , cyclic voltammetry (CV) measurements confirmhigher cathode and anodic currents for Pd- and Pt-containing CNTs overpristine CNTs and lower charge to discharge over-potential. PrisitneCNTs cathodes also had broader peaks. FIG. 11 shows the capacityretention due to cycling of the pristine CNT, Pd-filled CNT, andPt-filled CNT lithium sulfur batteries for the first 100 cycles.Referring to FIG. 11 , the capacity retention was improved when eitherPd or Pt nanocatalysts were embedded in the CNTs. Lithium sulfurbatteries containing Pd-filled CNT cathodes or Pt-filled CNT cathodesretained almost double the capacity of batteries containing pristine CNTcathodes after 100 cycles of full charge and discharge.

In an embodiment, a lithium sulfur battery can comprise an anode, acathode, and an electrolyte disposed between the anode and the cathode,where the cathode comprises: a conductive layer (e.g., an aluminum,copper, silver, or gold layer such as an aluminum, copper, silver, orgold foil) coated with a sulfur source; and a carbon structure having acatalyst filled therein without a surface coating of the catalyst on thecarbon structure. The catalyst can include, for example, nanoparticlesof a platinum group metal (e.g., at least one of ruthenium, rhodium,palladium, osmium, iridium, and platinum). For example, the catalyst canbe a palladium nanoparticle catalyst. The carbon structure can includeat least one of graphene, fullerenes, amorphous carbons, and carbonnanotubes (e.g., multi-walled carbon nanotubes). The carbon structurecan be coated on the conductive layer. The sulfur source can comprise,for example, elemental sulfur, lithium sulfide, lithium polysulfide, ora combination thereof. The lithium sulfur battery can further comprise aseparator (e.g., a polyolefin separator such as a polypropyleneseparator) disposed between the anode and the cathode, the electrolytebeing soaked in the separator.

In an embodiment, a lithium sulfur battery can comprise an anode, acathode, and a separator including an electrolyte and disposed betweenthe anode and the cathode, where the cathode comprises: a conductivelayer (e.g., an aluminum, copper, silver, or gold layer such as analuminum, copper, silver, or gold foil) coated with a sulfur source; amulti-walled carbon nanotube coated on the conductive layer; and acatalyst filled in an annular cavity of the multi-walled carbon nanotubewithout a surface coating of the catalyst on the multi-walled carbonnanotube. The catalyst can be, for example, a palladium nanoparticlecatalyst. The electrolyte can include a lithium salt. The lithium sulfurbattery can further comprise a conductive current collector (e.g., astainless steel rod) disposed on the cathode and/or a conductive currentcollector (e.g., a stainless steel rod) disposed on the anode. Theseparator can be a polyolefin separator (e.g., a polypropyleneseparator), and/or the electrolyte can be soaked in the separator.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

What is claimed is:
 1. A lithium sulfur battery, comprising: an anode; acathode an electrolyte disposed between the anode and the cathode; and aseparator disposed between the anode and the cathode, the cathodecomprising: a conductive layer coated with a sulfur source; and a carbonstructure having a catalyst filled therein without a surface coating ofthe catalyst on the carbon structure, the catalyst having nanoparticlesof a platinum group metal, and the carbon structure being coated on theconductive layer.
 2. The lithium sulfur battery according to claim 1,the platinum group metal including at least one of ruthenium, rhodium,palladium, osmium, iridium, and platinum.
 3. The lithium sulfur batteryaccording to claim 1, the catalyst being a palladium nanoparticlecatalyst.
 4. The lithium sulfur battery according to claim 1, the sulfursource comprising at least one of elemental sulfur, lithium sulfide, andlithium polysulfide.
 5. The lithium sulfur battery according to claim 1,the electrolyte being soaked in the separator.
 6. The lithium sulfurbattery according to claim 1, the separator being a polyolefinseparator.
 7. The lithium sulfur battery according to claim 1, theconductive layer being an aluminum foil.
 8. The lithium sulfur batteryaccording to claim 1, the electrolyte including a lithium salt.
 9. Thelithium sulfur battery according to claim 1, further comprising: a firstconductive current collector disposed on the cathode, the firstconductive current collector being porous; and a second conductivecurrent collector disposed on the anode.
 10. The lithium sulfur batteryaccording to claim 9, the first conductive current collector being astainless steel tube, and the second conductive current collector beinga stainless steel rod.
 11. The lithium sulfur battery according to claim1, the catalyst being a palladium nanoparticle catalyst, the sulfursource comprising at least one of elemental sulfur, lithium sulfide, andlithium polysulfide, the electrolyte being soaked in the separator, theseparator being a polyolefin separator, the conductive layer being analuminum foil, the electrolyte including a lithium salt, the lithiumsulfur battery further comprising: a first conductive current collectordisposed on the cathode, the first conductive current collector beingporous; and a second conductive current collector disposed on the anode,the first conductive current collector being a stainless steel tube, andthe second conductive current collector being a stainless steel rod. 12.A lithium battery, comprising: an anode; a cathode; and a separatorincluding an electrolyte and disposed between the anode and the cathode,the cathode comprising a carbon cloth gas diffusion layer, amulti-walled carbon nanotube coated on the carbon cloth gas diffusionlayer, and a catalyst filled in an annular cavity of the multi-walledcarbon nanotube without a surface coating of the catalyst on themulti-walled carbon nanotube, the catalyst comprising nanoparticles of aplatinum group metal, the lithium battery being a lithium sulfurbattery, and the cathode further comprising a sulfur source.
 13. Thelithium battery according to claim 12, the platinum group metalincluding at least one of ruthenium, rhodium, palladium, osmium,iridium, and platinum.
 14. A lithium battery, comprising: an anode; acathode; and a separator including an electrolyte and disposed betweenthe anode and the cathode, the cathode comprising a carbon cloth gasdiffusion layer, a multi-walled carbon nanotube coated on the carboncloth gas diffusion layer, and a catalyst filled in an annular cavity ofthe multi-walled carbon nanotube without a surface coating of thecatalyst on the multi-walled carbon nanotube, the catalyst comprisingnanoparticles of a platinum group metal, and the catalyst being apalladium nanoparticle catalyst.
 15. The lithium battery according toclaim 12, the separator being a polypropylene separator, and theelectrolyte being soaked in the separator.
 16. The lithium batteryaccording to claim 12, the electrolyte including a lithium salt.
 17. Thelithium battery according to claim 12, further comprising: a firstconductive current collector disposed on the cathode, the firstconductive current collector being porous; and a second conductivecurrent collector disposed on the anode.
 18. The lithium batteryaccording to claim 17, the first conductive current collector being astainless steel tube, and the second conductive current collector beinga stainless steel rod.
 19. A lithium sulfur battery, comprising: ananode; a cathode; a separator including an electrolyte and disposedbetween the anode and the cathode; a first conductive current collectordisposed on the cathode, the first conductive current collector beingporous; and a second conductive current collector disposed on the anode,the cathode comprising a carbon cloth gas diffusion layer, amulti-walled carbon nanotube coated on the carbon cloth gas diffusionlayer, a sulfur source, and a catalyst filled in an annular cavity ofthe multi-walled carbon nanotube without a surface coating of thecatalyst on the multi-walled carbon nanotube, the catalyst comprisingnanoparticles of a platinum group metal, the platinum group metalincluding at least one of ruthenium, rhodium, palladium, osmium,iridium, and platinum, the separator being a polypropylene separator,the electrolyte being soaked in the separator, the electrolyte includinga lithium salt, the first conductive current collector being a stainlesssteel tube, and the second conductive current collector being astainless steel rod.